Selenium, Omega-3s May Stave Off Colorectal Cancer

TUESDAY Dec. 8, 2009 -- Certain dietary supplements appear to affect the development of colorectal cancer or its recurrence, two new studies suggest.

In one study, researchers from the U.S. National Institute for Environmental Health Sciences found that eating a diet high in omega-3 fatty acids cut the risk of developing colorectal cancer by nearly 40 percent. In the other study, from cancer researchers in Italy, consumption of a dietary supplement containing selenium was found to reduce the chances of having polyps recur by a similar amount.

Both studies were to be presented Dec. 7 in Houston at a conference on cancer prevention sponsored by the American Association for Cancer Research.

In the selenium study, 411 people, 25 to 75 years old, who'd had one or more colorectal polyps removed took either a supplement or a placebo. The supplement, described as an antioxidant compound, contained 200 micrograms of selenomethionnine (a combination of selenium and methionnine), 30 milligrams of zinc, 6,000 international units of vitamin A, 180 milligrams of vitamin C and 30 milligrams of vitamin E.

Participants had a colonoscopy one year, three years and five years after starting the regimen.

Polyps recurred in 4.2 percent of those taking the supplement, compared with 7.2 percent of the placebo group. Overall, the study found, people taking the supplement had about a 40 percent reduction in risk for a return of polyps.

The researchers estimated that, after 15 years, about 48 percent of those taking the supplement would still be free of polyps, versus about 30 percent of those not taking the supplement.

Polyps, or adenoma, are benign growths on the large bowel. Though only a small proportion progress to become cancer, about 70 to 80 percent of colorectal cancer cases begin as polyps, according to the American Association for Cancer Research. About one in four people, most older than 60, will have at least one adenoma.

Selenium is found in soil, and human consumption comes by eating plants that have absorbed the nutrient or fish or animals that have eaten plants as part of their diet. "The content of selenium in the food depends on the soil content of this trace element, and in the same country there are areas at high, adequate or low content of selenium in the soil," said the study's lead author, Dr. Luigina Bonelli, head of the unit of secondary prevention and screening at the National Institute for Cancer Research in Genoa, Italy.

Earlier research had suggested that selenium can inhibit cell proliferation in the colon and rectum, Bonelli said.

Michele Forman, a professor of epidemiology at the M.D. Anderson Cancer Center in Houston, said that, though the findings are interesting, it's impossible to tell if the benefit was attributable to the selenium or to the other vitamins and minerals included in the supplement.

"You really don't know if it's the selenium or some combination that reduces risk of recurrence," Forman said.

In addition, the daily dosages of vitamins A and E taken by the participants were higher than the recommended daily allowances, Forman added. High levels of such vitamins can be detrimental, she said.

In the omega-3 study, U.S. researchers surveyed 1,509 whites and 369 blacks about their dietary habits in the past year. About half of the participants had colorectal cancer.

Among the white participants, those whose diets were in the highest fourth of omega-3 fatty acid consumption were 39 percent less likely to have colorectal cancer than those in the lowest fourth. However, for reasons the authors said they did not know, no association was noted between omega-3s and a reduction of colorectal cancer risk among black participants. The disease occurs at a higher rate among blacks than whites.

"Our finding clearly supports the evidence from previous experimental and clinical studies showing that long-chain omega-3 fatty acids inhibit tumor growth," said the study's lead author, Sangmi Kim, a postdoctoral fellow at the U.S. National Institute of Environmental Health Sciences in Research Triangle Park, N.C.

Kim said the research supports boosting omega-3 intake through diet or perhaps by taking an omega-3 supplement. Omega-3 fatty acids are found in fish, especially oily fish such as salmon, mackerel, herring, anchovies, sardines and tuna. Plant-based sources include flax and flaxseed oil, Brussels sprouts, soybeans and soybean oil, canola oil, spinach, walnuts and kiwi.

Previous studies have suggested that omega-3 fatty acids act as anti-inflammatory agents and help prevent cancer. But in the new study, Forman noted, participants were asked about their diets after they had been diagnosed with colorectal cancer so it's possible that their recollections were not fully accurate.

In addition, she said, it's possible that the benefit was not the result of omega-3s. Those who ate more fish might have had a healthier diet overall, she said.

"Were they eating a salmon-and-broccoli diet or a hamburger-and-french-fry diet?" Forman asked. "We don't know enough to say that it's truly the effect of the omega-3s."